1. Field of the Invention
This invention relates generally to inexpensive high strength, flexible, thin, improved electrical connectors and supports for tubular solid oxide electrolyte fuel cells in a fuel cell generator.
2. Description of the Prior Art
Square pitched, series-parallel, nickel felt to nickel coated interconnection components of solid oxide fuel cells are well known, and taught in U.S. Pat. Nos. 4,490,444 and 4,833,045 (Isenberg and Pollack-Reichner, respectively). Their connection to the main fuel cell generator current collect bus bars was also taught by Isenberg, in U.S. Pat. No. 4,648,945. The fuel cells used usually contain a self-supported air electrode tube, where the air electrode is covered over about 300 degrees by a solid electrolyte film. Thus, there is a 60 degree wide axial strip down the length of the cell. This remaining 60 degrees of air electrode surface is covered by an interconnection strip, usually a nickel plated lanthanum-chromite. As a top layer, fuel electrode covers the solid electrolyte over about 280 degrees of the electrolyte surface.
These cylindrical cells are usually placed in a square pitch, series-parallel connected array, wherein the air electrode of one cell is connected to the fuel electrode of the adjacent series-connected cell by a plated interconnection strip and a strip of 80% to 95% porous sintered nickel felt, which is about 0.1 inch (0.25 cm) thick. Other nickel felts provide parallel connections between the fuel electrodes of adjacent cells. The series path is essential for the generation of a practical-DC stack voltage. The parallel connections provide paths by which the current can circumnegotiate any defective open circuit cells. Fuel flows axially in the passages between the groups of cells. This has been one tubular fuel cell design for many years.
In this design, the primary subassemblies from which a solid oxide fuel cell generator is formed are called “cell bundles”. Usually, cell bundles contain twenty-four cells on an 8×3 cell matrix. Eight cells are series connected to form one row of a three-row bundle. The three rows are connected in parallel through the connection of each cell in the row with the adjacent cell in the next row. Between the nickel plated interconnection strip of one cell and the nickel fuel electrode of the next cell in a row, any two cells are presently series connected by a nickel felt of a rectangular cross-section (approximately 10 mm×14 mm). Parallel connection is also currently accomplished by similar felt strips. In this case, the felts connect the fuel electrodes of adjacent cells. Along the length of a cell, eight felts of about 185-mm length are used to form a series connection, and four felts of 185-mm length are used to accomplish a parallel connection. A total of 280 felt strips are used per bundle. This means of electrical connection is effective; however, it is costly in terms of materials and is labor intensive. Furthermore, this arrangement is not very conducive to automation.
Improvements to this standard design have been suggested. U.S. Pat. No. 5,273,838 (Draper/Zymboly) eliminated one nickel felt connector from each group of four cells, where alternate cells of a first row had no electrical connection of their interconnections to cells in an adjacent row. This design helped to eliminate the potential for bowing when using newer, longer one meter cells. This design may, however, decrease the overall strength of the twenty-four cell subassemblies.
In an attempt to simplify generator design and reduce assembling costs, DiCroce and Draper, in U.S. Pat. No. 5,258,240, taught a thick, flat-backed, porous metal fiber felt connector strip, having a crown portion of metallic fiber felt conforming to the surface of its contacting fuel cell These porous felt connectors could be used as a series of thin strips across a small part of the fuel cell length, or as a porous sheet extending along the entire axial length of the fuel cells. In order to provide structural integrity, since there are no side connections, a plurality of cells would have to be laminated to provide a thickness of 0.125 inch (0.62 cm), thereby reducing porosity to about 5 to 10%. The strips could also be made of a solid nickel foil or a composite of foil and porous felt; they could also have two opposing fuel cell conforming surface, as shown in FIG. 3 of that patent. The use of fibrous felts still allowed potential densification during prolonged use. Additionally, it was difficult to fashion such felts to exact dimensions, and the felts retailed a springiness. Conversely, the use of foils did not provide adequate strength, and prevented the required infiltration of he bundle with hot air during the drying process, which is an important feature of bundle manufacture.
Draper et al. in U.S. Pat. No. 6,379,831 B1 attempted to solve all these problems by providing a corrugated mesh electrical connector having a top crown and bottom shoulder portion where the mesh between fuel cells could be straight to impart rigidity or, as shown in FIG. 5 of that patent, the mesh between fuel cells was also corrugated, but in all cases, the nickel felt connectors were completely eliminated by direct connection of the crown portion to the nickel coated interconnection, dramatically reducing the number of parts to assemble each bundle. This design while inexpensive requires extremely high quality connector electroplating. And also results in less than desirable physical contact with the nickel plating of the interconnection, so that any given force that is applied to the screen/nickel plated interconnection joint results in very high localized stresses at the points of contact between the mesh and the plating.
In a completely unrelated area, metal foams have been used as a heat exchange media, as taught be D. P. Haack et al. in “Novel Lightweight Metal Foam Heat Exchangers”, 2001 ASME Congress Proceedings, New York, November 2001; and as fuel cell components for water management, heat exchange, flow plates and catalyst substrate for reformers as described at www.porvairfuelcells.com, allowing faster transfer of heat energy than in ceramic structures. Metals used include platinum, copper, steel, nickel, silver, cobalt, rhodium and titanium, among others. Ceramic foam filters have also been taught in U.S. Pat. No. 5,456,833 (Butcher et al.)
What is needed is a highly porous nickel based electrical connector/support to conform to and support all contacting fuel cells, as well as to connect all contacting fuel cells electrically, where connector to nickel plated interconnection contact strength is much improved and where electrical conductivity at the same contact point is also much improved. The connector/support must be strong, but it must also be possible to increase even more the desired flexibility by selection of an appropriate combination form or shape, without use of metal felts.